1. Field of the Invention
The present invention relates to a printing method for forming a pattern of a liquid crystal display and, more specifically, to a solvent for printing.
2. Discussion of the Related Art
Ultra-thin flat panel displays include a display screen with a thickness of not more than a few centimeters. Of these, liquid crystal displays are currently used in a wide range of applications, such as notebook computers, monitors, spacecrafts, and aircrafts, in terms of advantages of low power consumption due to low operating voltage and ease of portability.
Such liquid crystal displays include a lower substrate, an upper substrate, and a liquid crystal layer interposed between the two substrates. Generally, thin film transistors and pixel electrodes are formed on the lower substrate, and a light-blocking layer, a color filter layer and a common electrode are formed on the upper substrate.
That is, liquid crystal displays include various constituent elements, and a number of processing steps are repeated to form the constituent elements. Particularly, photolithography has been employed to pattern the constituent elements into various shapes.
However, since photolithography requires the use of a patterned mask and a light irradiation apparatus, it has the disadvantage of increased fabrication costs. In addition, since photolithography involves light exposure and development processes, it is disadvantageous in terms of complicated processing procedure and long processing time.
Thus, there exists a need for a novel patterning method for overcoming the disadvantages of photolithography. Printing has been introduced to satisfy the need.
Printing is a process wherein a pattern composition is applied to a printing roll to form a desired pattern and then the pattern is transferred to a substrate by rotating the roll to accomplish the formation of the pattern on the substrate.
Such printing process is simple and economically advantageous when compared to conventional photolithography processes. Hereinafter, a conventional printing process will be explained in more detail with reference to the accompanying drawings.
FIGS. 1a to 1c are process diagrams schematically showing the procedure of a conventional patterning method by printing.
First, as shown in FIG. 1a, a pattern composition 20 is applied to a blanket 35 attached to a printing roll 30 through a printing nozzle 10.
The blanket 35 is required to have firm attachment to the printing roll 30 and no deformation during printing. As a material for the blanket satisfying these requirements, a Si-based resin is widely used.
Then, as shown in FIG. 1b, the printing roll 30 is rotated on a printing plate 40 having protrusions 45 with a predetermined shape to transfer a portion 20a of the pattern composition 20 to the protrusions 45 of the printing plate 40. A residual portion of the pattern composition leaves a pattern 20b having a predetermined shape on the blanket 35 of the printing roll 30.
Then, as shown in FIG. 1c, the printing roll 30 is rotated on a substrate 50 to transfer the pattern 20b to the substrate 50, completing formation of the desired pattern.
To sum up, the conventional printing process is carried out by applying the pattern composition 20 to the blanket 35 attached to the printing roll 30 and sequentially transferring the applied pattern composition 20 to the printing plate 40 and the substrate 50 to form a pattern. The characteristics of the pattern composition 20 determine whether the application and transfer steps are well performed or not.
That is, the pattern composition is a solution of a pattern material (for example, a BM material or a pigment for a color filter) in a solvent. Failure in the selection of an optimum solvent leads to inaccurate control of the application and transfer steps, making it impossible to form a fine pattern. The reasons will be explained in more detail below.
Firstly, when the pattern composition 20 is non-uniformly applied to the blanket 35 of the printing roll 30 in the application step shown in FIG. 1a, accurate pattern formation is not ensured.
Secondly, when the pattern composition 20 applied to the blanket 35 after the application step shown in FIG. 1a is not dried within a short time (i.e. when a solvent contained in the pattern composition is not evaporated within a short time), the transfer of the pattern composition 20 is poorly performed, leading to formation of an inaccurate pattern.
FIG. 2a shows the occurrence of defects during transfer in the case where the pattern composition 20 applied to the blanket 35 is not dried within a short time. As is evident from region “A” shown in FIG. 2a, the undried pattern composition 20 is not completely transferred to the printing plate 40 and hence accurate pattern formation is not achieved.
Thirdly, when the pattern composition 20 applied to the blanket 35 after the application step shown in FIG. 1a is excessively dried (i.e. when an excess amount of the solvent is evaporated from the pattern composition 20 to increase the viscosity of the pattern composition to a specified level or higher), the transfer of the pattern composition 20 is poorly performed in the transfer steps shown in FIGS. 1b and 1c, which leads to formation of an inaccurate pattern.
FIG. 2b shows the occurrence of defects during transfer in the case where the pattern composition 20 applied to the blanket 35 is excessively dried. As is evident from region “B” shown in FIG. 2b, when the viscosity of the pattern composition 20 is increased to a specified level or higher, the pattern composition 20 may be transferred to unwanted regions and hence accurate pattern formation is not achieved.
Fourthly, when the blanket 35 is dissolved in a solvent present in the pattern composition 20 after the application step shown in FIG. 1a, swelling of the blanket 35 occurs, leading to formation of an inaccurate pattern.
As explained above, the problems caused in the application and transfer steps of the printing process are attributed to the pattern composition 20, particularly, the solvent contained in the pattern composition for printing. Consequently, selection of an optimum solvent is essential in the formation of an accurate pattern.
Until now, however, no solvent has been developed that can overcome the above problems.